Surface phenomena during plasma-assisted atomic layer etching of SiO\u3csub\u3e2\u3c/sub\u3e

\u3cp\u3eSurface phenomena during atomic layer etching (ALE) of SiO\u3csub\u3e2\u3c/sub\u3e were studied during sequential half-cycles of plasma-assisted fluorocarbon (CF\u3csub\u3ex\u3c/sub\u3e) film deposition and Ar plasma activation of the CF\u3csub\u3ex\u3c/sub\u3e film using in situ surface infrared spectroscopy and ellipsometry. Infrared spectra of the surface after the CF\u3csub\u3ex\u3c/sub\u3e deposition half-cycle from a C\u3csub\u3e4\u3c/sub\u3eF\u3csub\u3e8\u3c/sub\u3e/Ar plasma show that an atomically thin mixing layer is formed between the deposited CF\u3csub\u3ex\u3c/sub\u3e layer and the underlying SiO\u3csub\u3e2\u3c/sub\u3e film. Etching during the Ar plasma cycle is activated by Ar\u3csup\u3e+\u3c/sup\u3e bombardment of the CF\u3csub\u3ex\u3c/sub\u3e layer, which results in the simultaneous removal of surface CF\u3csub\u3ex\u3c/sub\u3e and the underlying SiO\u3csub\u3e2\u3c/sub\u3e film. The interfacial mixing layer in ALE is atomically thin due to the low ion energy during CF\u3csub\u3ex\u3c/sub\u3e deposition, which combined with an ultrathin CF\u3csub\u3ex\u3c/sub\u3e layer ensures an etch rate of a few monolayers per cycle. In situ ellipsometry shows that for a ∼4 Å thick CF\u3csub\u3ex\u3c/sub\u3e film, ∼3-4 Å of SiO\u3csub\u3e2\u3c/sub\u3e was etched per cycle. However, during the Ar plasma half-cycle, etching proceeds beyond complete removal of the surface CF\u3csub\u3ex\u3c/sub\u3e layer as F-containing radicals are slowly released into the plasma from the reactor walls. Buildup of CF\u3csub\u3ex\u3c/sub\u3e on reactor walls leads to a gradual increase in the etch per cycle.\u3c/p\u3